1. Field of the Invention
The present invention relates to a semiconductor device having a solder layer on the rear electrode thereof, and a method o manufacturing such a semiconductor device. The present invention can be effectively adopted to a semiconductor device forming a power device therein.
2. Description of the Related Art
Consecutive sputtering of titanium, nickel and gold layers onto a semiconductor substrate or wafer to form aminated metal electrode, wherein an argon atmosphere having a pressure in the range of 2 to 10 mTorr is used during the sputtering, is generally known. This pressure range is adopted because, at a higher argon pressure, the electric resistivity of the deposited layer is high and the load on a vacuum pump becomes higher as the argon pressure is increased.
Nevertheless, in the conventional sputtering, nigh stresses occur particularly in the deposited nickel layer, thereby bending the semiconductor wafer and reducing the adhesive strength of the deposited layers.
(Prior Work)
To solve such a problem, the technique described in Japanese Patent Publication HEI 2-167890 was proposed. The technique is that the sputtering is carried out in an argon gas atmosphere wherein the pressure of the argon gas as a discharge gas is at least 15 mTorr. This forms a nickel layer having a reduced stress. The resultant ratio of the X-ray diffraction peak intensity of the (200) plane of the nickel layer to that of the (111) plane of the nickel layer is at least 10%. The nickel layer reduces the bending of the wafer and enhances the adhesive strength because of the reduced stress.
However, when the argon pressure is high like the aforementioned technique, the density of the nickel layer becomes sparse as shown in FIG. 6. Therefore, when a solder layer for connecting a rear electrode is formed on this nickel layer, the diffusion speed of tin (Sn) and lead (Pb) in the solder layer into the nickel layer is increased, and the adhesive strength of the solder is reduced.
FIG. 7 shows a measured useful life of the semiconductor in relation to the argon pressure used during sputtering. The solder used in the measurement consists of 40% tin (Sn) and 60% lead (Pb) by weight. As shown in FIG. 7, when the argon pressure is above 15 mTorr, the useful lifetime is rapidly shortened.
We believe that the following model explains the reason why the useful life is required. FIG. 8 shows an auger electron spectroscopy depth profile of a peel-off surface of a sample having a solder layer consisting of 40% tin and 60% lead by weight. The sample has a titanium (Ti) layer, a nickel (Ni) layer and a gold (Au) layer between a silicon substrate of the sample and the solder layer. The solder layer is soldered on a lead frame. The peel-off surface appeared when the silicon substrate was peeled off the lead frame. As understood from the result of this analysis, the peel-off surface of the sample is formed between a titanium (Ti) layer, which is formed in between a silicon substrate and the nickel layer, and lead of the solder.
Lead exists in tin as a solid solution. Tin having lead therein diffuses in and passes through the nickel layer during a high-temperature exposure test, and tin arrives at a interface between the titanium layer and the nickel layer. Because it is difficult for lead to diffuse in the titanium layer, lead is collected at the interface, and therefore, the strength of this portion is weakened.
When a semiconductor device having the aforementioned structure is adopted as a device for a motor vehicle, the reduced of strength due to the diffusion of the solder is a severe problem because the environmental temperature encountered by devices for motor vehicles is high (more than 100.degree. C.).